Process for depositing a protective aluminum coating on metal articles



Dec. 30. 1969 G M. c. A. GAUJE PROCESS FOR DEOSITING A PROTECTIVEALUMINUM COATING ON METAL ARTICLES Filed Feb. l5, 1966 gunzuma.4

United States Patent O 3 486,927 PROCESS FOR DEPOSITING A PROTECTIVEALUMINUM COATING ON METAL ARTICLES Georges Maurice Clestin Alos Gauje,Paris, France, aS-

signor to Societe Nationale dEtude et de 'Construction de Moteurs'dAviation, Paris, France, a company of France Filed Feb. 15, 1966, Ser.No. 527,598 Claims priority, application France, Feb. 16, 1965, 7

U.S. Cl. 117-107.2 8 Claims ABSTRACT F THE DISCLOSURE A process fordepositing a protective aluminum coating on a metal article whichcomprises submitting, at a high temperature, said article to the actionof an aluminum halide vapor, said vapor being formed by reacting at ahigh temperature an aluminum alloy remaining in the solid state at saidtemperature with a halogen. The aluminum alloy may comprise from 6 to50% by weight of aluminum and said temperature may be between 950 andThe present invention relates to a process for depositing a protectivealuminum coating on a metal article made of a refractory alloy, saidcoating being deposited in the vapour phase and being intended inparticular to protect the metal articles from oxidation and from gaseouscorrosion at high temperatures.

Such protection is in particular sought after in various heat enginesthe eiciency of which is conditioned by their maximum intaketemperature, this latter being limited by the behaviour of theconstituent materials of the articles that are most exposed to heat andto constraints. The invention is particularly applicable, though notexclusively so, to certain turbo-engine elements or parts, such ascombustion chambers, fixed or moving blades belonging to the turbine,thermal shields, elements belonging to the jet pipes, etc. Said partsare most often made of refractory alloys, being either forged or castwith a base of nickel or cobalt, generally with considerable proportionsof chromium, molybdenum, tungsten, etc. added. Said alloys possessexceptional dimensional rigidity at high temperatures, and this allowsthem to manifest only limited deformations under the effects ofmechanical and thermal constraints, but on the other hand theirbehaviour in the event of oxidation and corrosion by hot gases is veryinadequate and does not allow satisfactory exploitation of the very goodcreep strength of these alloys at high temperatures, hence the need toprotect said refractory alloy parts from oxidation and from oxidationand corrosion by hot gases by cladding the surface of the parts with ametal or an alloy that gives rise to a protective coating either throughthe formation of oxides that are stable at the working temperature ofsaid parts or through the establishment of a diffusion barrier for theoxygen. and the other corrosive elements of the hot gases. Saidprotective coating should, moreover, possess good adhesion to the basealloy and should not scale olf under the action of deformations ofmechanical or thermal origin.

Very varied processes for deposition have been employed with theabove-stated aim most of them bringing about the diffusion of the metalto be deposited at a high temperature and in a halogenated atmosphere,the metal of addition and the part to be covered being in contact.

According to the present invention, the metal parts, more especiallybeing made of refractory alloy with a base of nickel or cobalt and thatare to be clad with a protective coating, are subjected, in anappropriate 3,486,927 Patented Dec. 30, 1969 chamber heated, to a hightemperature, to the action of a halide (volatile at the temperature ofsaid chamber) of the metal to be deposited, which volatile halide can.be diluted with an inert or reducing gas and is formed by the reactionat a high temperature of a suitable substance on a donor substance whichremains solid at the reaction temperature and which contains the metalto be deposited, preferably alloyed with at least one of the metalswhich constitute the refractory alloy to be protected, in such a mannerthat the metal to be deposited is transferred to the parts whilst Whollyin a gaseous phase, without solid-to-solid contact between the donor andsaid parts. The donor can be employed again and again, the halogenelement content being regenerated as the need arises.

As regards the etliciency of the protection, it is preferable to choosealuminium as the metal of addition by reason of the excellent protectivecoatings it provides on the surface of the refractory alloy as itcombines with it. It is in this sense preferable to other metals, suchas chromium for example, because it imparts to the protected articles aconsiderably longer life than does the latter metal. For example, forcomparison purposes, it has been found that a cobalt-base refractoryalloy known for having a quite marked sensitivity to oxidation andcontaining approximately 60% of cobalt, 20% of chromium, 10% oftungsten, 0.5% of carbon and various elements, and protected :by `acoating of chromium deposited in a halogenated atmosphere, manifests anoverall gain of 50 to 60 grams per square metre at the termination of acycle that comprises, in all, four successive periods of 24 hours at1,100" C. in an oxidising atmosphere, the periods being broken by briefreturns to the ambient temperature, this being aimed at obtainingthermal shock effects. The same alloy, protected according to theinvention by a deposit of aluminium, manifests after the same test again in weight of under 10 grams per square metre. The simultaneousaddition of aluminium and of chromium, or, indeed, of aluminium,chromium and silicon, or any other multiple addition of elements thatare known for their ability to protect metals and alloys from oxidationand corrosion at high temperatures, does not yield results superior tothose obtained by the addition of aluminium alone. Excellent behaviorhas been obtained, in an oxidising environment, by the surface coatingof alloy articles by aluminium alone, in conformity with the presentinvention.

The process of the present invention permits the obtaining, on thesurface of the articles, protective coatings that are actually bound tothe base metal as a result of the formation of an alloy between the basealuminum and the metal deposited, this occurring without the articles tobe protected being subjected to any chemical attack, and correspondinglyto any deformation. The protective coatings obtained have smoothsurfaces free of local excrescences, a very important point because ithas been verified that, apart from geometric and aerodynamic faults thatthese can bring about, these irregularities correspond to abruptvariations in the local concentration of the metal of additon. TheseIdifferences in surface coating, when linked to rough patches, lead tounequal resistances to oxidation and corrosion. For these reasons theprocess of the invention is particularly advantageous because it allowsa smooth and appreciably uniform deposit to be obtained whatever theshape of the article to be coated.

The following description, referring to the annexed drawings and givenby way of non-limitative example, will explain how the invention may becarried into effect, the further features emerging both from the textand the drawings also forming part of said invention.

FIGURE 1 is a diagrammatical view in vertical crosssection of oneembodiment of the process of the invention.

FIGURE 2 is a diagrammatical view in vertical crosssection of anotherembodiment of the process of the invention.

In FIGURE 1, a chamber 1, the shape of which depends upon that of thefurnace 2 which is employed and upon that of the articles to be coated,is provided with a lid 3, which does not require to be leakproof. Thearticles 4 to be protected are suspended from a stand 5 in such lamanneras to be at a distance from the donor substance 6. Said donor isseparated from the space containing the articles requiring coating bywalls 7 that includes perforations. Thus any solid-to-solid contactbetween the articles 4 and the donor 6 is avoided. The distance betweenthe articles 4 requiring coating and the donor alloy charge 6 may varyfrom one to several centimetres without the thickness and the quality ofthe deposit obtained being affected. The conveying of the protectingelement from the donor alloy charge 6 to the articles 4 is effectedsolely through the intermediary of a gas, that is, a halide of the metalto be deposited, which halide is formed during the heating process.

The donor 6, which must remain in a solid state at the reactiontemperature, is an alloy containing, as its first main constituent, oneor more of the base elements of the alloy to be coated, for instance,cobalt, nickel, chromium, iron, etc., and as its second constituentaluminum. It is obvious that no mention need be made of conventional andeveryday impurities introduced by commercial supplies of said materials.The content of aluminum in the donor may vary, for example, from 6 to50%, 'and is preferably between 30 and 40%.

The donor may be in any form, from a very fine powder to lumps.

One of the particular advantages of the present invention is that, dueto the employment of a donor as described above, there is no needed toadd an inert compound, such for example as alumina in granular form, tothe aluminium-base donor, in order to avoid the agglomeration of thelatter. Such an addition would be highly disturbing because it wouldimpede the free circulation of the gases and the active participation inthe reaction of the whole mass of the donor placed in the chamber.

Once the charge has been constituted as indicated above, `a halogen orhalogenated compound is introduced, preferably fluorine or chlorine in agaseous state, or a combination of halogen, for example uorine andchlorine, together with aluminum to be coated. The amount of the halogenor halogenated compound introduced, in relation to the total mass of thedonor, is not critical and may vary, for example, from 0.1 to 1% -byweight.

The chamber 1, provided in this manner with its donor, its halogenatedelement and its articles that require coating, is placed in the furnace2, the atmosphere of which may be constituted by a neutral gas such :asargon, or by a reducing medium such as hydrogen or cracked ammonia.

The heating temperature is regulated in such a manner las to bring abouta sufficient vapour pressure 0f the halide of aluminum to be deposited(a halide formed by a reaction when hot between the halogen and thedonor within the chamber 1), as well as a quite deep simultaneousdiffusion, into the articles 4, of aluminum deposited on their surfacethrough the decomposition of such .aluminum halide.

The duration of the heating period at the chosen temperature allows boththe thickness of the deposit, and the content of aluminum in the surfacecoating alloy so formed, to be regulated in a very precise manner.

As an indication, the temperature may vary between 950 and 1200 C.approximately, and the practical duration for the maintenance of thistemperature may lie between 1 and 5 hours approximately.

The determination of the thickness of the deposit is effected to afforda compromise, in use, `between a duration of protection in the presenceof heat and a resistance to scaling at the time of thermal shockeffects, both duration and resistance being as great as possible. Ingeneral, the thickness of the deposit will lie between 0.025 and 0.100millimetres, and preferably between 0.04 and 0.07 millimetres.

Numero-us successive coating operations may be carried out whilecontinuing to employ the same charge. Experience shows, indeed, that inevery case the same thickness of deposit is obtained, everything inother respects remaining equal. It is Sufficient merely to regenerate inpart the halogenated element; in actual fact, this latter may escape byreason of the chamber lid leaking, and it is therefore necessary tomaintain its concentration at an adequate level. Said concentration ismoreover weak, as has been shown above, given tha-t the halogen elementsonly serve to convey aluminum to -be deposited and `are continuallyybeing regenerated in the course of the reactions.

In the embodiment shown in FIGURE 2, the single chamber of FIGURE 1 isdivided into two distinct chambers 1 and 8.

The chamber 8, intended to receive the donor 6, is placed in a furnace9; said chamber is provided at two of its opposite ends with a gas inlet10 having two pipes 11 and 12 and with a gas outlet 13. The gas outlet13 allows the chamber 8 to communicate with the chamber 1 and issurrounded either with heat insulation or with a heating device 14.

The chamber 1, placed inside the furnace 2, encloses the stand 5 for thearticles 4 that are to be coated and includes a gas outlet 15 as well asa loading door with an appropriate seal 16 that ensures an adequatefreedom from leaks while allowing easy loading and unloading of thechamber.

The halogen (fluorine or chlorine, for example )is introduced in theform of a gas stream at the inlet 10 of the chamber 8 by means of thepipe 12. Said halogen may ybe mixed with a neutral or reducing gas(argon, hydrogen, etc.) called the carrier gas, so as to effect asuitable dilution and at the same time ensure a forward propulsion ofthe gases. Said gases, more or less rich in gaseous halogen, pass overthe donor 6 that is placed in the chamber 8, this being heated to atemperature sufficient for the production of a volatile halide to takep-lace. The gas stream then passes into the second chamber 1 to be putin contact with the articles to be coated that are contained therein,this chamber 1 being raised to a temperature sufficient for thedecomposition of the aluminum halide to be 'brought about upon contactwith the articles to be coated and for the metal,A once deposited, todiffuse into the articles, from the surface towards the heart of saidarticles.

The pipe 13 is arranged in such a manner that the temperature prevailinginside it is sucient to avoid any premature reaction or decomposition inthe middle of the gaseous mixture circulating therein; in general, thistemperature is at least equal to that in the chamber 8.

The carrier gas and the excess aluminum halide finds an outlet by way ofthe pipe 15 and pass through suitable absorbers that allow said excessto be re-cycled if so required, with the aid of industrial processesthat are well known and are not here described. By way of example, thetemperature of the chambers may be maintained between 950 and l200 C.approximately, the temperature of the first chamber 8 being lower thanthat of the second chamber 1.

lObviously numerous modifications may be applied to this embodimentwithout, however, going beyond the scope of the present invention, asdefined by the appended claims. In particular, the arrival of thecarrier gas may be effected at the outlet to the chamber 8 instead of atits inlet. Similarly, the position of the two chambers is only given byway of non-limitative example, both as regards their relative positionand as regards the vertical or horizontal position of either chamber.

The articles made yof refractory alloy that have been protectedaccording to the process of the invention behave excellently for severalhundred hours at temperatures of the order of ll C.

What is claimed is:

1. A process for protectively coating, with aluminum substantiallyalone, a metal article made of a refractory alloy, which comprisesdisposing the article which is to be protectively coated, in a chamberheated to a temperature comprised between about 950 and l200 C.;providing this chamber with an aluminum halide which is volatile at thetemperature of said chamber, said volatile aluminum halide being formedby the reaction, at a temperature comprised between about 950 and 1200C., of a halogen with a donor substance being out of solid-to-solidcontact with said article and consisting of an aluminium alloy whichremains in the solid state at the reaction temperature; and allowingsaid volatile aluminium halide to decompose when contacting said articlewhereby a protective aluminium coating is deposited on said articlewithout there being any chemical reaction of said metal article withsaid volatile halide.

2. A process as claimed in claim 1 wherein the donor substance is analloy containing aluminum and at least one of the metals constitutingthe refractory alloy of the article.

3. A process as claimed in claim 1 wherein the donor substance is analloy containing aluminum and at least one metal selected from the groupconsisting of cobalt, nickel, chromium and iron.

4. A process as claimed in claim 1 wherein the content of aluminum inthe donor substance ranges between 6% and 50% by Weight.

5. A process as claimed in claim 1 wherein the content of the aluminumin the donor substance ranges between 301% and by weight.

6. A process as claimed in claim 1 wherein the halogen is selected fromthe group consisting of fluorine and chlorine.

7. A process as claimed in claim 1 wherein the metal article issubmitted to the action of the aluminum halide for a period of between 1and 5 hours.

8. A process as claimed in claim 1 wherein the halogen is.. diluted withan inert gas.

References Cited UNITED STATES PATENTS 1,770,177 7/1930 Martin.1,814,392 7/1931 Low et al. 23-87 2,816,048 12/1957 Galmiche 117-1072.2,856,312 x10/1958 Nowak et al. 117-1072 X 3,079,276 2/1963 Puyear etal.

OTHER REFERENCES Powell et al.: Vapor Plating, 1955, pages 3, 4, 25 and261 relied upon.

ANDREW G. GOLIAN, Primary Examiner U.S. C1. X.R. 117-130

